The human erythrocyte is chosen as a model system to study the process of cellular aging. In the absence of a nucleus, the cell apparantly undergoes time-dependent alterations during a relatively short life-span of 120 days, resulting in the loss of cellular deformability and eventual senescence and removal from the circulation. Cells are obtainable in large quantities, and are readily separable by age-related difference in their density. Chemically, the proteins and lipids of the red cell membrane are well characterized. The spectrin-actin cytoskeleton situated on the cytoplasmic surface of membrane is believed to be an important determinant of cell shape and deformability. We plan to use both biochemical and biophysical approaches to determine the structural interactions among these cytoskeletal-contractile proteins and their role in cell shape regulation and cell aging. The biochemical approach includes studies on the binding properties among spectrin, actin and band 4.1 and between cytoskeletal and integral membrane proteins. The biophysical approach includes physical characterization of binary and ternary complexes of cytoskeletal proteins by viscometry and light scattering techniques, as well as measurement of membrane lipid fluidity by electron paramagnetic resonance. With these studies, we hope to establish a correlation between the state of cytoskeletal proteins and the age-related rigidity of the cell membrane.